Medical equipment for radiation therapy treats tumorous tissue with high energy radiation. The dose and the placement of the dose must be accurately controlled to insure both that the tumor receives sufficient radiation to be destroyed, and that damage to the surrounding and adjacent non-tumorous tissue is minimized.
Internal-source radiation therapy places capsules of radioactive material inside the patient in proximity to the tumorous tissue. Dose and placement are accurately controlled by the physical positioning of the isotope. However, internal-source radiation therapy has the disadvantages of any surgically invasive procedure, including discomfort to the patient and risk of infection.
External-source radiation therapy uses a radiation source that is external to the patient, typically either a radioisotope, such as .sup.60 Co, or a high energy x-ray source, such as a linear accelerator. The external source produces a collimated beam directed into the patient to the tumor site. External-source radiation therapy avoids some of the problems of internal-source radiation therapy, but it undesirably and necessarily irradiates a significant volume of non-tumorous or healthy tissue in the path of the radiation beam along with the tumorous tissue.
The adverse effect of irradiation of healthy tissue may be reduced, while maintaining a given dose of radiation in the tumorous tissue, by projecting the external radiation beam into the patient at a variety of "gantry" angles with the beams converging on the tumor site. The particular volume elements of healthy tissue, along the path of the radiation beam, change, reducing the total dose to each such element of healthy tissue during the entire treatment.
The irradiation of healthy tissue also may be reduced by tightly collimating the radiation beam to the general cross section of the tumor taken perpendicular to the axis of the radiation beam. Numerous systems exist for producing such a circumferential collimation, some of which use multiple sliding shutters which, piecewise, may generate a radio-opaque mask of arbitrary outline.
As part of collimating the beam to the outline of the tumor, the offset of the radiation beam, with respect to a radius line between the radiation source and the center of rotation of the radiation source, may be adjusted to allow the treated area to be other than at the center of rotation. Simultaneously changing the offset and the width of the radiation beam as a function of gantry angle allows tumorous tissue having an irregular cross-section within a plane parallel to the radiation beam to be accurately targeted. The width and offset of the radiation beam may be controlled by the use of a multiple-leaf circumferential collimator.
Adjustment of the offset and size of the radiation beam at various gantry angles allows considerable latitude in controlling the dose. Nevertheless, even using these techniques, there is still a considerable amount of undesired dose imparted to healthy tissue, especially where a treatment volume is concave or highly irregular within the plane parallel to the radiation beam.
A radiotherapy machine providing much reduced irradiation of healthy tissue is presented in co-pending U.S. patent application Ser. No. 07/865,521, by Stuart Swerdloff et al, filed Mar. 19, 1992. That application discloses the use of a number of radiation attenuating leaves in a rack positioned within the radiation beam before the beam enters the patient. The leaves slide into the radiation beam in a closed state and out of the radiation beam in an open state to allow unobstructed passage of a given ray of the beam ray. By controlling the ratio of time spent in the open and closed states, each ray may be attenuated over a continuous range of intensities. This ability to control not just the outline of the radiation but the intensity of each individual ray allows extremely precise control of the irradiation volume.
The radiation attenuating compensator described above must be capable of completely attenuating every ray of the fan beam. A single uncontrolled ray that leaks past the compensator to a patient will provide undesirable exposure to organs and other tissue including tissues outside a targeted treatment volume. Passage of unattenuated rays is avoided by using leaves with finely machined adjacent faces that can pass very close to each other minimizing the gap size between adjacent leaves minimizing leakage.
However, in practice, regardless of accurate machining techniques, small gaps between the leaves, that allow unattenuated rays to pass, are needed to prevent frictional contact. The exacting tolerances necessary to minimize the size of the leaf gaps are costly and such high tolerance components are prone to failure.
Another problem with the compensator described above is that the rays are not uniformly attenuated by the moving leaves because the leaves cannot be moved instantaneously. A leaf initially occludes the entire depth of its associated ray within the beam. As the leaf begins to move out of the beam, part of the ray is occluded and another part is left unobstructed. Eventually, the entire ray is unobstructed. The same non-uniform beam attenuation is again encountered when the leaf moves back into the beam.
This gradation in attenuation can be minimized by equipping the compensator with more powerful actuators to drive the leaves in and out of the fan beam width more rapidly. Bigger actuators, however, are more costly to utilize and maintain. Alternatively thinner leaves may be used which are light weight so that they can be moved more quickly. This, however, demands more actuators, creates more leaf gaps, and must be accommodated by a much more complicated control system.